The present invention relates generally to manufacturing processes, and, more specifically, to laser shock peening.
The strength of a metal workpiece may be improved by introducing compressive residual stress in the external surface thereof. Shot peening is one conventional process to introduce such residual compressive stress.
Another process uses the high energy of an industrial laser beam to burn an ablative coating on the workpiece within a confinement layer such as water which causes an instantaneous explosion, and the corresponding introduction of plastic deformation in the workpiece surface for introducing the residual compressive stress. In such laser shock peening (LSP) the laser operates in a pulse mode, with laser beam pulses being traversed across the surface of the workpiece for introducing the residual stress therein.
The continuing development of the LSP process includes splitting the laser beam into two opposite beams which strike opposite surfaces of the workpiece for particular advantage. Since the main beam from the laser generator is split in two, its two components simultaneously impact the opposite sides of the workpiece. However, the two split beams require precise alignment with the workpiece to ensure that the simultaneous impact occurs at oppositely aligned spots on the workpiece for increasing efficiency of operation.
One exemplary workpiece which may benefit from the use of the LSP process is the airfoil commonly found in gas turbine engines in the compressor or turbine sections thereof. A typical airfoil has a concave pressure side and an opposite convex suction side joined together at chordally opposite leading and trailing edges, and extending in radial span from a root to a tip. At the root is a platform which defines the inner flow boundary for the airfoil, and an integral dovetail may extend below the platform for removably mounting the airfoil blade into the corresponding slot of a supporting rotor.
Each engine includes many rows of such airfoils in the compressor and turbine sections thereof, and LSP is required for the full complement of airfoils in each row as desired. Accordingly, the LSP process must be repeated to cover the substantial external surface of each airfoil, and then repeated for the multiple airfoils for each rotor stage. The corresponding manufacturing time accordingly increases the cost of the process.
The process necessarily begins with an alignment procedure to ensure that the two opposite laser beams are aligned with the opposite sides at the same location of an individual airfoil. The airfoil itself is suitably mounted in a fixture which is attached to the distal end of a conventional, multiaxis computer numerically controlled (CNC) robot or manipulator. The manipulator includes a computer controller which is suitably programmed in software for controlling the desired movement of the workpiece airfoil relative to the stationary laser and the beams emitted therefrom. In this way, the workpiece is precisely moved in three-dimensional space for traversing the laser beams in a predetermined path over the surface of the airfoil to effect complete laser shock peening thereof, which is simultaneous for both sides of the airfoil.
Although the pressure and suction sides of the airfoil are curved in the typical manner, the two sides generally face oppositely to each other which permits relatively quick alignment of the two laser beams generally opposite with each other at about 180 degrees. A conventional alignment procedure has been used for over a year in this country to prepare the manipulator for the LSP processing of airfoil workpieces which have been sold and are found in commercial use.
The alignment process includes a conventional alignment fixture in the form of a rectangular beam having a transverse through hole in the distal end thereof. Two annular cover plates are mounted in the common hole through opposite sides of the fixture, with each plate including a center aperture transversely aligned with the opposite aperture. The two plates have corresponding internal circular seats which adjoin each other along the longitudinal center plane of the fixture, and support two target sheets trapped inside the through hole.
The fixture may then be attached to the distal end of the manipulator which is programmed to position the through hole and the aligned apertures thereat at the focal or intersection point of the opposite laser beams. In this way, the laser beams may be directed through the corresponding opposite apertures in the alignment fixture to mark the target sheets in the alignment procedure. The target sheets may be formed of a suitable material such as photographic or bum paper, which will produce a visible mark when exposed to the laser beam, typically produced by operating the high power LSP laser at a suitably low power setting.
The corresponding burn marks on the target sheets may then be examined and measured for any misalignment therebetween. The two laser beams should be aligned within a few mils of each other, and any measured discrepancy thereof may be suitably adjusted by adjusting the alignment of the laser beams using the conventional mirror adjustments found therein.
The alignment procedure may be repeated one or more times as desired to confirm the accuracy of alignment of the opposing laser beams relative to their intersection points on the alignment fixture. The alignment fixture may then be simply removed and replaced by the workpiece, such as an airfoil mounted to the manipulator on a suitable supporting fixture. The manipulator is then suitably programmed to position the airfoil with its opposite sides facing respective ones of the two laser beams so that the LSP process may be simultaneously effected on opposite sides of the airfoil with accurate alignment of the two impact sites. The manipulator then moves the mounted airfoil in three-dimensional space so that the two laser beams may traverse the external surfaces of the airfoil for laser shock peening thereof.
As indicated above, an individual airfoil for a gas turbine engine has curved pressure and suction sides which generally face oppositely to each other so that the oppositely aligned laser beams may simultaneously impact the opposite sides of the airfoil at substantially the same location in space. The airfoils typically twist from root to tip, yet the opposite external surface thereof may still be suitably aligned with the two laser beams by rotating the airfoil along its span axis to reposition the local sites of the airfoil between the opposite laser beams.
However, such twisting airfoils may be integrally formed with the supporting rotor in a unitary or one-piece blisk assembly. In an exemplary compressor blisk, the full row of airfoils extends radially outwardly from the supporting rotor with a relatively close spacing around the circumference thereof, with the individual airfoils nesting between the next adjacent airfoils. The adjacent airfoils in a blisk therefore prevent the use of laser beams aligned oppositely about 180 degrees apart due to the blocking effect thereof.
Accordingly, the LSP process requires that the two laser beams be realigned at an included angle substantially less than 180 degrees, and even down to a small acute included angle as low as about 20 degrees. In this way, the two laser beams may be directed to the opposite sides of an individual airfoil in a compressor blisk to avoid the obstruction of the next adjacent airfoils in the blisk.
In this configuration of the oblique laser beams, the initial alignment thereof becomes more complex. Since the conventional alignment fixture in the form of a rectangular beam has a small but substantial thickness, and the target apertures in the distal end thereof extend transversely through the fixture, the fixture itself introduces self-obstruction with the oblique laser beams particularly at small or acute included angles therebetween.
In order to effectively use the conventional alignment fixture with the oblique laser beams, one of the two cover plates is removed for removing the self-blocking effect thereof, and the target sheets are simply taped into the exposed through hole against the remaining cover plate. The so-modified alignment fixture is then conventionally used in the alignment procedure, with the oblique laser beams having elliptical projections on the target sheets due to the relative inclination therewith.
The elliptical laser beam projections increase the difficulty of aligning the opposite beams, and the alignment process requires iteration by replacing the marked target sheets with clean sheets again taped into the fixture hole. However, taping and untaping of the target sheets lacks accuracy or repeatability of location and further complicates the alignment procedure.
The alignment procedure for the oblique laser beams can therefore require up to about a half a day which is a substantial expenditure of time, which is typically repeated each and every day of the laser shock peening process for ensuring accuracy thereof. The alignment procedure therefore increases the overall time for laser shock peening the multitude of workpieces, and correspondingly increases the cost of manufacture.
Accordingly, it is desired to provide an improved laser shock peening target for reducing time of alignment of oblique laser beams.